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LoadAndDecodeL1Entry will now always return the size/alignment of the
entry. This allows ConstructCAMfromPageTables to walk over a 2MB long
descriptor page table pointer in one go, instead of splitting it into
two 1MB chunks (as if short descriptor page tables were in use) and
calling LoadAndDecodeL1Entry twice. This has allowed the 1MB result
alignment bodge to be removed from the LongDesc version of
LoadAndDecodeL1Entry.

This changes PhysRamTable to store the address of each RAM bank in terms
of (4KB) pages instead of bytes, effectively allowing it to support a 44
bit physical address space. This means that (when the long descriptor
page table format is used) the OS can now make use of memory located
outside the lower 4GB of the physical address space. However some
public APIs still need extending to allow for all operations to be
supported on high RAM (e.g. OS_Memory logical to physical address
lookups)

OS_Memory 12 (RecommendPage) has been extended to allow R4-R7 to be used
to specify a (64bit) physical address range which the recommended pages
must lie within. For backwards compatibility this defaults to 0-4GB.

This adds initial support for the "long descriptor" MMU page table
format, which allows the CPU to (flexibly) use a 40-bit physical address
space.

There are still some features that need fixing (e.g. RISCOS_MapInIO
flags), and the OS doesn't yet support RAM above the 32bit limit, but
this set of changes is enough to allow for working ROMs to be produced.

Also, move MMUControlSoftCopy initialisation out of ClearWkspRAM, since
it's unrelated to whether the HAL has cleared the RAM or not.